Retroviruses have been and are being used currently as gene delivery vehicles for introducing desired genes into cells, and are being used in a number of gene therapy trials. (Miller, Nature, Vol. 375, pgs. 455-460 (1992)). Cornetta, et al., Progress in Nucleic Acid Research and Molecular Biology, Vol. 36, pgs. 311-322 (1990) and Cornetta, et al., Human Gene Therapy, Vol. 1, pgs. 15-30 (1990) teach the intravenous infusion into monkeys of an amphotropic murine leukemia retrovirus which includes the human ADA gene. Clinical illness in the monkeys was not observed after administration of the retrovirus. The retroviruses were cleared rapidly from the circulation, and such clearance was mediated in part by complement, which inactivates the virus.
In some trials, direct gene delivery to cells in vivo is being undertaken; for example, the delivery of the Herpes Simplex Virus thymidine kinase gene by recombinant retroviruses to tumors. (Oldfield, et al., Human Gene Therapy, Vol. 4, pgs. 39-69 (1993)). For these trials, murine leukemia virus amphotropic strain (MLV-A) packaging cells, constructed in murine NIH 3T3 cells, are being used to produce recombinant retroviruses.
Murine leukemia viruses produced by NIH3T3 cells, however, can be inactivated rapidly by human serum. Such inactivation is a result of activation of the complement cascade.
The inhibition of infectivity of C-type retroviruses first was demonstrated in reports that four strains of murine leukemia viruses (MLV), and Moloney Sarcoma Virus (MSV) pseudotypes with the envelope specificity of gibbon ape leukemia virus (GALV), or simian sarcoma associated virus (SSAV), were inactivated by fresh, but not heated human serum. (Welsh, et al., Nature, Vol. 257, pgs. 612-614 (1975); Welsh, et al., Virology, Vol. 74, pgs. 432-440 (1976).) Lysis of these viruses, as well as lysis of feline leukemia virus (FeLV), the cat endogenous virus RD114, simian sarcoma associated virus (SSAV), the baboon endogenous virus M28 (BaEV) and the D-type virus Mason-Pfizer monkey virus (MPMV) by human serum was demonstrated by the release of reverse transcriptase activity from virions. (Welsh, et al., 1975; Welsh, et al., 1976; Sherwin, et al., Int. J. Cancer, Vol. 21, pgs. 6-11 (1978).) Complement depleted, or deficient human sera failed to cause viral lysis, and complement consumption was observed when viruses were added to human serum. (Welsh, et al., 1975). Murine leukemia viruses were shown to be lysed following direct, antibody independent triggering of the Clq component to MLV virions. (Cooper, et al., J. Exp. Med., Vol. 144, pgs. 970-984 (1976).) An isolated 15 Kda virion protein with a pI of 7.5, proposed to be the p15E transmembrane protein, was shown to trigger complement. (Bartholomew, et al., J. Exp. Med., Vol. 147, pgs. 844-853 (1978).) Retroviruses made by murine cells, however, were found to be inactivated by human serum mainly by recognition of sugar epitopes, Gal (1-3) Gal epitopes, by natural antibodies (Takeuchi, et al., unpublished data).